Phylum: Chlorophyta – green algae
British distribution (genus): Widespread, freshwater.
World distribution (genus): Throughout most of the world.
Pediastrum is a green alga forming flat, radially symmetrical, discoid colonies in freshwater. Species differ in the number of cells, always a power of two. The cells are thick-walled and commonly the marginal cells bear two horn-like projections, giving the colonies their characteristic appearance. Some species, such as that shown here, may have internal gaps between the cells.
Usually in eutrophic waters, in plankton and amongst aquatic macrophytes. Noted as sometimes forming blooms in arctic lakes. The cell walls are resistant to decay and may persist in lake sediments.
Photograph of material in culture, 1998, species unnamed but apparently P. boryanum var. cornutum.